1. Field of the Invention
This invention relates to a process for the production of chemically bonded non-woven sheet materials containing a binder with a microporous or microheteroporous structure. The sheet material formed by the process has extremely advantageous properties rendering it particularly suitable for use as a carrier for synthetic leather.
2. Description of the Prior Art
It is known that the demand for leather for use in the footwear and garment industries as well as material for technical products has vastly increased and this has caused a significant increase in the manufacture and use of synthetic leather. The use and application of synthetic leather products are particularly significant in the footwear industry, in the vehicle manufacturing industry and in the industry manufacturing decorative leather products.
It is also known that the use of synthetic leather in substitution of natural leather makes high demands on the production of synthetic leather in that in its appearance, touch or feel, workability and applicability it should provide a leather effect.
To satisfy these demands, it is known to provide synthetic leathers with a carrier designed to provide predetermined properties in the end product. From this it follows that it is a significant task to ensure that the characteristics of the carrier material meet the desired requirements relating to synthetic leather. This means that the carrier material itself must ensure the advantageous properties of the synthetic leather. Thus, the task of the carrier material can be designated as increasing mechanical strength properties, satisfying the expectations of mechanical and physical properties, a favorable degree of flexibility or bendability of the synthetic leather as well as softness or plasticity, and organoleptic behavior. Taking into account all of these factors, it is particularly significant that the microheteroporosity of the binder has an important role in fulfilling these expectations.
It is known that for chemically strengthening and for manufacturing non-woven sheet materials, the following processes are employed in general:
A non-woven sheet material containing a binder of microheteroporous structure can be manufactured by impregnating a needled and shrunk fiber fleece with a solution of a polymer or a mixture of polymers, e.g., a polyurethane dissolved in dimethylformamide, then the polymer is coagulated in a liquid solvent which does not dissolve the polymer (water-miscible solvent), e.g., water, see British Patent Specification No. 1,091,935 and U.S. Pat. Nos. 3,676,206, 3,483,283, 3,067,482 and 3,228,786.
A disadvantage of this process is that it requires the use of solvents which are harmful to humans and which are inflammable, explosive and its cost are high. The process is also disadvantageous from the point of view of environmental pollution. Because of these reasons, the manufacture of products made by this process has not been adopted on a wide scale.
A further process is known wherein the needle-punched and shrunk fiber fleece is impregnated with an aqueous dispersion of polymers which, after destabilization of the dispersion, is dried and hardened or cured, see British Patent Specification No. 1,273,311, U.S. Pat. Nos. 3,539,388, 3,639,146, 3,523,059 and 3,228,786.
The process operating with an aqueous dispersion is free from the technological disadvantages and other harmful effects mentioned above. Hence, the use of such a process is more advantageous and is in wide use. However, it is to be considered a significant disadvantage that the non-woven sheet material product of the aqueous dis- persion processes contains a binder material which is not of microheteroporous structure, and therefore, its properties and characteristics do not meet certain requirements.